1. Field of the Invention
This invention relates to a mechanism for preventing the operation of a circuit interrupting device during the time that an excessive overcurrent flows therethrough, and, more particularly, to a ferromagnetic mechanism, operated by flux generated by the excessive overcurrent, for preventing movement of a movable contact thereof during the occurrence of excessive overcurrents which exceed the interrupting device's interrupting rating.
2. Prior Art
Many types of circuit interrupting devices are well known in the art. Such devices include fuses or fuse-like devices, circuit switchers or similar devices, and circuit breakers. Generally speaking, these devices have several common features. Specifically, most circuit interrupting devices typically include an insulative housing with opposed circuit-connectable terminals thereon. Within the housing is usually contained a movable contact, often called an "arcing rod," and a stationary contact. During circuit interruption the movable contact or arcing rod is moved away from the stationary contact to elongate an arc formed therebetween. Arc elongation usually takes place in the vicinity of an arc extintuishing medium, which may be a solid material such as boric acid or a fluid such as sulfur hexafluoride. Arc elongation plus the action of the arc extinguishing medium ultimately result in extinguishment of the arc at a current zero to interrupt the circuit.
Prior art circuit interrupting devices generally have at least two significant electrical ratings. The first significant rating is the continuous or normal current-carrying rating or capability of the device; the second is the current interrupting rating or capability of the device. Again, generally speaking, should the device be called on to interrupt a current in excess of its interrupting rating, undesirable consequences may follow. For example, the device may violently explode, burn or melt.
Specific differences between the various types of circuit interrupting devices usually involve their manner of sensing overcurrents, the manner in which the moving contact or arcing rod is moved away from the stationary contact and the type of arc-extinguishing medium used. In fuses, or fuse-like devices, a fusible element may be utilized. Usually an assembly, which includes the fusible element, prevents movement of the arcing rod by a stored energy mechanism, often a spring. Melting of the fusible element in response to an overcurrent flowing through the device permits the spring to effect the above-described movement of the arcing rod. See for example, commonly assigned U.S. Pat. Nos. 4,103,270 and 4,075,755, and commonly assigned U.S. patent applications Ser. No. 909,144, filed May 24, 1978, and Ser. No. 741,027, filed Nov. 11, 1976, all incorporated by reference hereinto. Recently developed fuse-like devices include electronic means for sensing current conditions within the circuit. Should an overcurrent be sensed, an appropriate signal is sent to a power cartridge or similar facility for ignition thereof. Ignition of the power cartridge operates a piston-cylinder arrangement to move the arcing rod. Examples of such recently developed devices may be found in commonly assigned U.S. patent applications, Ser. No. 909,145 and Ser. No. 909,146 both filed May 24, 1978 and incorporated by reference hereinto. Circuit switchers and similar devices rely on electronic or electrical sensing to provide a trigger signal to an electro-mechanical operator. Operation of the operator effects movement of the moving contact within the circuit switcher. Circuit breakers similarly often include electronic or thermal elements which sense overcurrents to operate electrical or electro-mechanical apparatus to ultimately effect movement of the moving contact.
As noted above, extreme overcurrents in excess of the current interrupting rating of prior art circuit interrupting devices may occur. If the device is called on to interrupt such currents, it may fail in that task. Should such happen, the circuit must ultimately be protected by upstream protective devices with which the circuit interrupting device is coordinated. However, failure of the circuit interrupting device, even for a very short time, to perform its interrupting task may ultimately lead to destruction of, or damage to, the circuit including the device itself and electrical apparatus and structures in its vicinity.
Accordingly, it is an object of the present invention to permit circuit interrupting devices to operate normally when they interrupt currents within their current interrupting rating but to prevent their operation during the occurrence of overcurrents which exceed that rating, relying solely on upstream protective devices to interrupt the circuit. In this way, the circuit interrupting device is not subjected to a violent failure and portions thereof may be subsequently reused. Moreover, the risk of damage to the circuit and to structures or electrical apparatus in the vicinity of the circuit interrupting device is minimized.
The present invention represents a departure from the circuit interrupting device depicted in U.S. Pat. No. 2,871,415. In that device an overcenter toggle mechanism operates separable contacts. The device is provided with a first operating solenoid having both an armature connected to one lever of the toggle mechanism and trip and close windings which are suitably energized to effect either separation or closing of the contacts. A second solenoid is provided which has an armature and a holding winding. The holding winding is connected in electrical series with the contacts, and the armature of the second solenoid is adjustably connected to the armature of the first solenoid so as to restrain and prevent tripping of the interrupting device at predetermined different maximum values of current.
The present invention relates to a mechanism, the end result of which is similar to the device of the U.S. Pat. No. 2,871,415 but which is simpler to use, less expensive and more positive in operation.